1. Field of the Invention
This invention relates to magnetic recording media, such as the media used with hard disk drives, and in particular to servo sectors on the magnetic recording media. Still more particularly, the present invention relates to a servo sector format for a patterned media.
2. Description of the Prior Art
Designers, manufacturers, and users of computing systems require reliable and efficient digital information storage and retrieval equipment. Conventional magnetic disk drive storage systems are typically used and are well known in the art. As the amount of information that is stored digitally increases, however, users of magnetic recording media need to be able to store larger and larger amounts of data. To meet this demand, designers of magnetic recording media are working to increase the storage capacity of a recording disk, which is a function of the number of closely spaced concentric tracks on the surface of the disk. Some of the recording disk surface area, however, must be used for purposes other than data storage.
Some systems use various types of indexing marks and alignment indicia to help keep the head properly aligned on a particular track. The indexing marks and alignment indicia are used for precise position reference and track adjustment of the read/write head. These marks and indicia are often recorded in servo sectors, which are angularly-spaced reserved portions of the recording disk surface that extend out approximately radially from the disk centers. Track addresses and angular synchronization signals that determine the circumferential location of the magnetic head can also be recorded in servo sectors. Normal and quadrature position error signal (PES) bursts are often recorded in servo sectors for generation of position error signals that are used to keep the read/write head aligned. Servo sectors use recording disk surface area that could otherwise be used for data storage. Consequently, servo sector information must be stored as efficiently as possible in order to leave as much space as possible for data storage.
Servo sectors are typically written with a servo track writer (STW). The servo track writer provides a clock head that is inserted into the drive and a radial pusher mechanism to accurately position the disk drive actuator. The disk drive to be servo written is placed on the STW, a clock track is written, the actuator arm is positioned at various radial locations, and the desired servo pattern is written.
Ideally, this servo writing process would produce tracks that form perfect concentric circles about the center of rotation of the disk spindle. The tracks would also be spaced at a desired track pitch across the disk. Track pitch is defined as the distance between the centerlines of the track, and in an ideal recording disk the track pitch is equal between each individual track. Unfortunately, factors such as mechanical vibrations that are asynchronous to disk rotation during the servo writing process, along with disk defects and edge/transition noise cause the tracks to form irregular concentric paths and generate deviations in track pitch.
The STW process described above is known in the industry as an open loop STW method because the pusher does not follow the disk flutter and spindle runout. To correct for the errors created during the servo writing process, the disk drive servo is used to estimate the written errors of the servo sectors. Correction factors for each servo sector are then written immediately after each servo sector. The estimation process is one of statistical averaging in which the error is reduced by a factor of F by reading the servo sectors on each track F*F times (F squared). Hence, the amount of time is takes to write a drive is proportional to the cube of the number of tracks N. In other words, if it takes N revolutions to write the tracks, it will take N*N revolutions to correct for the written errors. Consequently, the servo sector information should be recorded on the disk as quickly as possible.
One limitation to conventional servo track writing is the side fringe is fields that emanate from the write heads. When a write head is writing data on a track, the fringe fields can erase data that is stored on tracks adjacent to the one being written. Furthermore, the fringe fields can cause the written magnetic data marks to have curved ends. Magnetic data marks are best written with straight, radial edges because it makes it easier for the head to read the data marks. Thus, it is undesirable to have curved ends on the data marks.
Another limitation to conventional servo systems is the fact that they require high sampling rates due to the high track density seen in contemporary recording disks. But some hard disk drives, such as desktop drives, spin at speeds having low revolutions per minute (RPM). As a result, the low RPM disk drives need more servo sectors per revolution than high RPM disks in order to ensure the servo system in the low RPM drive obtains an adequate sampling rate. Unfortunately, having more servo sectors greatly reduces the format efficiency of a disk. In other words, there is less space for data storage in the low RPM drives.
Given the time consuming and potentially inaccurate process for conventional STW, fabricating the servo sector using a printing process such as the process used in the fabrication of CDs and DVDs is sometimes used. This process is called imprint lithography. It consists of a precision stamper that is mastered on a precision optical or electron beam rotary positioning system. The printed pattern defines both the circumferential and radial edges of the sector servo bits. However, the format pattern used in the imprint lithography process is limited by the size of the optical or electron beam generated by the patterning tool. The beam size limitation may make it impossible to make the servo bits as short as they should be for minimizing the size of the servo sector for a given track density. This means that patterning the servo bits may result in a poor servo format efficiency.
Still another limitation of contemporary servo patterned media is that the edges of the bits have to be aligned at an accurate angle with respect to the play back head in the drive and that the distance between leading and trailing edges have to be precisely controlled. This additional accuracy makes the mastering of the patterned media disk difficult and may make the manufacturing yield low.
Other limitations that result from the use of patterned media are that the surface topography of the media may affect the flying of the recording head over the disk surface and may require a thicker protective overcoat to prevent corrosion of the recording films. It is therefore desirable to make the spatial frequency low and to make the size of the patterns as small as possible.
What is needed is a way to pattern a magnetic recording media that results in a compact servo sector having a high servo format efficiency.
In accordance with the present invention, a servo sector format for a patterned media is disclosed. The servo sector format is comprised of a first patterned servo timing mark, a short patterned gray code patterned adjacent to the first servo timing mark, a plurality of PES burst separators patterned adjacent to the gray code, and a second patterned timing mark patterned adjacent to the plurality of separators.
After the disk is patterned, a plurality of PES burst islands defined by the PES burst separators are DC magnetized at the inner diameter (ID). In the preferred embodiment, the adjacent corners of the first two PES islands form a single normal (NINIT) null pattern bit when DC magnetized. The third and fourth islands form a single quadrature (QINIT) null pattern bit when DC magnetized. The NINIT and QINIT null pattern bits are used to start a self-initialization process in which tones are written on all of the PES islands and in which all other areas of the disk are DC erased.
The resulting plurality of servo burst fields preferably form a quad. pattern comprised of an A burst, a B burst, a C burst and a D burst. After the initial servo burst field patterns are written, a recording head whose writer is offset radially to the outer diameter (OD) from the reader is used to perform the self-initialization process. The reader tracks on the previously written servo burst fields and the writer writes tones on the A, B, C and D PES islands to the outer diameter of the previous written tones. The normalized A-B amplitude signal is then used to generate the normal PES13N for the disk drive. The normalized C-D amplitude signal is used to generate the quadrature PES13Q for the disk drive.
The preferred embodiment further comprises magnetically written gray code written adjacent to the second patterned servo timing mark. The magnetically written gray code is preferably written circularly around the rotational center of the disk because the disks will typically be mounted in the disk drive such that the servo pattern is eccentric to the rotational center. The magnetically written gray code can be written at a higher spatial frequency than that of the patterned gray code and thus is more area efficient. The magnetically written gray code is used to define the track number of the data tracks with no eccentricity.
In the preferred embodiment, each magnetically written gray code bit is written by pulsing the recording head on the previously erased media. Two marks are written to encode each gray code bit. The marks are written on every other two-thirds of a track such that the effects of the head width variations and the fringe fields are eliminated. The magnetically written gray code bit is detected as the logical OR of the sensed signal in the two bit locations.
The first patterned servo timing mark indicates the start of a servo sector, and is preferably used when reading the patterned gray code and decoding the servo bursts. The patterned gray code is used in addressing the tracks when the magnetic gray code is self-written. The preferred gray code groups the tracks into groups of tracks that are a power of two in number so binary addressing can be used for the magnetic gray code. The plurality of separators are areas where no data can be written, which has the effect of masking the fringe fields of the writer. Finally, the second patterned servo timing mark acts as a check on the occurrence of the first servo timing marks, and is an accurate timing reference for the magnetic gray code transitions.